1. Field of the Invention
The present invention generally relates to a flat antenna apparatus, and especially relates to a flat antenna apparatus for UWB (ultra-wide band).
2. Description of the Related Art
In recent years and continuing, UWB radio communication technologies attract attention for their capabilities of RADAR positioning and large capacity transmission. Especially, since the approval by the U.S. FCC (Federal Communication Commission) in 2002 of UWB for public uses in a frequency band between 3.1 and 10.6 GHz, developments are being actively undertaken for utilization of UWB.
Since UWB uses a super-wide band, the antenna apparatus for UWB must be capable of super-wideband transmission and reception.
An antenna for use at the FCC approved 3.1-10.6 GHz band proposed by Non-Patent Reference 1 includes a ground plane and a feeder.
FIG. 1A and FIG. 1B show conventional antenna apparatuses 10 and 20, respectively. The antenna apparatus 10 includes a ground plane 11 and a feeder 12 that is shaped like a reversed circular cone provided on the ground plane 11. The side face of the circular cone shape of the feeder 12 has an angle θ to the axis of the circular cone. By adjusting the angle θ, a desired characteristic is acquired.
The antenna apparatus 20 includes a feeder 22 in the shape of a teardrop, configured by a circular cone 22a and a sphere 22b inscribing the circular cone 22a; the feeder 22 is arranged on the ground plane 11.
[Non-Patent Reference 1]
“An omnidirectional and low-VSWR antenna for the FCC-approved UWB frequency band” by T. Taniguchi and T. Kobayashi (Tokyo Denki University) in 2003 IEEE AP-S International Symp., volume: 3, pp. 460-463, Jun. 22-27, 2003. (Disclosure on March 22 at B201 classroom).
[Patent Reference 1] JPA 2000-196327.
The conventional antenna apparatuses tend to require a great volume because of the feeder of the circular cone or the teardrop being arranged on the ground plane; accordingly, miniaturization and a thinner shape are desired.
FIG. 2 shows a UWB flat antenna apparatus 30 disclosed by JPA 2005-160286 filed by the applicant hereto.
The UWB flat antenna apparatus 30 includes a substrate 31 made from dielectric material, the substrate 31 having an upper surface 31a and a bottom surface 31b. On the upper surface 31a, an antenna element pattern 32 and a line 33 (the line 33 including line sections 33a, 33b, 33c, and 33d) are formed. The line 33 extends from the antenna element pattern 32 that is shaped like a home base. Further, a three-stage ring filter 34 consisting of ring filter elements 35, 36, and 37 is formed between the corresponding line sections 33a, 33b, 33c, and 33d. Each of the ring filter elements 35, 36, and 37 has a stub. On the bottom surface 31b a ground pattern 38 is formed. The antenna element pattern 32 and the ground pattern 38 are closely arranged in a longitudinal direction of the substrate 31.
As compared with the conventional antenna apparatuses 10 and 20 shown in FIGS. 1A and 1B, respectively, the UWB flat antenna apparatus 30 is miniaturized and thin.
Nevertheless, the ring filter 34 with stubs is structured by multiple flat ring filter elements with stubs, namely, a ring filter element 35 with a stub serving as the first stage, a ring filter element 36 with a stub serving as the second stage, and a ring filter element 37 with a stub serving as the third stage. For this reason, the length L of the UWB flat antenna apparatus 30 tends to be great, which makes it difficult to miniaturize the UWB flat antenna apparatus 30.